The present invention relates in general to the construction of large boilers, and in particular to a new and useful expansion joint which establishes a seal between the interior and the exterior of a boiler furnace and which includes a trough containing fluidizable sand or other suitable fluidizable granular materials.
Large fluidized bed combustors or large boilers of greater than about 600 megawatts, must be bottom and top supported. A prior art solution to this support problem includes the formation of an expansion joint in the boiler tubes which comprises a substantially U-shaped portion of each tube projecting outwardly from the furnace walls by 6 to 8 feet. However, this awkward and expensive approach may not perform satisfactorily.
U.S. Pat. No. 2,840,043 to Durham teaches the use of a seal at the bottom of a boiler furnace which includes a flange depending from a wall of the furnace, and the flange extends into liquid held in a container which is supported on a refractory enclosure. While water is used as the liquid for establishing the seal in Durham, this is only appropriate where the water would not be exposed to excessive heat. According to Durham heat is not collected in the region along the top border of the bottom supported boiler portion, the latter being lined with refractory brick which keeps the water cool. A relatively high water level is also necessary in Durham, for example 2 feet. This level is necessary to provide an adequate seal. It is noted that a substantial seal is required to prevent leakage during changes in furnace pressure.
The bed area of a utility size fluidized bed boiler (greater than about 600 megawatts) is large thereby making it difficult to top support the unit unless several small beds are stacked. Stacking the beds, however, leads to an increase in the overall boiler height, and requires a start-up compartment for each bed which adds to the overall area. In addition, start-up compartments with heating surface will require complex ductwork, while compartments without heating surface will increase overall bed area and also require complex ductwork. This will impose some limitations on the gas velocity and dimensions of the convection pass.
In order to couple a one level (ranch style) bottom supported bed with a top supported convection pass, it is necessary to use an expansion joint to allow for free movement of the expanding ends during the heating process of the boiler. The expansion joint must provide the means for expansion, maintain a good seal around the unit, be simple in structure, particularly around the corners, withstand the high furnace temperatures and occupy a small space around the boiler in the horizontal and vertical planes.
References which are relevant to the present invention are U.S. Pat. No. 1,912,043 to Sanders which shows a pulverized fuel fired furnace and its support structure; U.S. Pat. No. 3,893,426 to Bryers which shows a heat exchanger that utilizes an adjoining fluidized bed; U.S. Pat. No. 3,857,334 to Coulter which shows a furnace seal that includes a liquid containing trough; and U.S. Pat. No. 3,685,165 to Deve which shows the fluidization of sand for its reclamation, the sand being used for foundry core molding.